Photoelectric apparatus for spectrographic analysis



Feb. 17, 1948. A, w, FlsHER ETAL 2,436,104

PHOTOELEGTRIC APPARATUS FORA SPECTROGRAPHIC ANALYSIS Filed May ll, 19444 Sheets-Sheet 1 HT'TOPNEYJ.

GNN.

Feb. 17, 1948. A. w. FISHER Erm.

PHOTOELECTRIC APPARATUS FOR SPECTROGRAPHIC ANALYSIS Filed May 11, 1944 4Sheets-Sheet 2 .um mg www WITNESSES.

Feb. 17, 1948. A, w- F|$HER ETAL 2,436,104

PHOTOELECTRIC APPARATUS FOR SPECTROGRAPHIC ANALYSIS Filed May 1l, 1944 4Sheets-Sheet 5 m 7. ./:lO W

Feb. 17, 1948. A. w. FISHER ETAL PHOTOEKLECTRIC APPARATUS FORSPECTROGRAPHIC ANALYSIS 4 Sheets-Sheet 4 Filed May 1l 1944 MQW www wrr/vesses.

creases, so that quantitative Patented Feb. 17, 1948rno'rorinacrmcniriina'rvs For: srEcTRooRArmc-ANALYSIS 'Aiken 4W. Fisherburgh, Pa.,

and William A14?.. lWarren, lPitts assignors to lf'islxer ScientificCom-- pany, Pittsburgh, Pa., a corioratin of Penn- Asylvania ApplicationMay 1-1, 1944,-Serial No. 535,157

4 Claims.

This invention relates to spectrographlc analysis and to apparatustherefor.

Spectographic analysis involves the determination of the constituentsand amounts of oo n- Istituents of a material by measurement of thelamount of radiant energy which they emit when a sample of the materialis excited appropriately', usually thermally or electrically, or both.Such measurements involve separation of the energy emitted by theindividual constituents, or elements, by means of a spectrograph whichfin its customary form comprises a slit for selecting a beam of energyemitted by the excited sample, a light-dispersing element, usually aprismor grating, which disperses the beam of radiant energy into aspectrum and, sometimes with the aid of suitable lenses, brings it to afocus-on a surface called the focal surface. In accordance withcustomary practice the resultant spectrum is then photographedon a iilmdisposed along the focal surface.

The wave lengths of the radiant energy emitted by an excited sampledepend upon the elements present in the sample so that appropriateexamination of the photographed spectrum, as by comparison with standardspectrograms of pure elements or of samples of known composition, servesto identify the elements present in the sample. In general, andwithincertain limits, the average intensities of 4lines of any particular wavelength, due to excita-tion of a particular element in a sample, increaseas the percentage of that element in the sample inanalysis is likewisepossible. This involves the measurement of the average intensity of atleast one of the spectrum lines emitted by each element desired, i. e.,meas; urement of the optical density of characteristic spectrum linesphotographed on the emulsion. This may be accomplished by meansoa-photocell, as is known, the average intensity which produced themeasured line being then determined by referring the measured opticaldensity to a calibration curve relating intensity and optical densityfor the' particular emulsion and particular processing procedure used indeveloping and finishing the negative. The amount of that elementpresent in the sample can then be determined from data relatingintensity to content of lelement vdetermined from standard samples.

Such a method "of analysis is based upon the assumption that percentageconcentration of an element is directly. proportional to the density ofa particulars-photographed spectrum line yof that element, but factorsinvolved in that method introduce inaccuracies. Thusfthe density of aparticular lineis a, function not only of the percentage concentrationof the element which emits it,vbut also of (1) the total energy emittedby1thelsample, which may, and usually does, vary with .changes .inexcitation conditions, (2) the fraction -of the total energy traversingthe. optical system, which involves variations in the location of theexcited sample with respect to the optical'axis fof thel instrument,-(3)the time of eX- posureof the emulsion to the spectrum, (4) thesensitivity of .the emu1sDI1. which in turn varies Withntime,lterriperatureand humidity as well as with manufacturing methods, (5)the processing procedure, which 'is dependent upon such variables asdeveloper concentration, the time and temperature-of development, andothers.

The effects .of ythese variables may be reduced, but not eliminated, b yrelating the density of a line vproduced by a particular. element to thedensityof a lineproduced by an internal standard element, i. e., AoneYwhich has an essentially constant percentage invall the samples to vbeanalyzed. Even so, the-photographic method, thus briefly described andwhich has been standard, is openl to the further disadvantage thatitvrequires a considerable length of time so thatvit is poorly adaptedtosuch uses as control work, where rapid quantitative-determinations are*essential. This l follows from Y.the fact that the emulsions mustbevexposed, developed, fixed and dried. the opticalfdensities of theparticular lines must lthen be determined -from the negative, andfinally the concentrations of the particular elements of the sample'mustbe calculated. Likewise, it requires skilled manipulation and tediouscalculations that necessitate careful training and generally require ascientific background.

Because ofthe possibility of error and of the protracted vnature ofspectrographic determinations it would bedesirable, accordingly, to haveavailable anapparatus for spectrographic analysis which avoids thosedisadvantages of the photographic procedure.

An object of the present invention is to provide apparatus forspectrographic analysis which measures directly and accurately, Withoutthe consistent necessityA of photography or a similar intermediary, theenergy of spectrum lines produced by dispersion of 'energy radiated froman excitedsample.

, Another object is, to provide apparatus-for directly measuring by.means of photocells charac'- teristic spectrum lines ofrenergyradiatedfrom l plan view of the apparatus accenna an excited sample,which is capable of high accuracy. compensates for variations in theexciting system and other uncontrollable variables, and eliminates orrepresses spectrograph analysis.

Still another object is to provide anapparatus which is simple,applicable generally to all types of spectrographic persons of notechnical training. can be made fully automatic, and which occupies lessspace than the conventional forms of photographic apparatus for thispurpose. y

A further object is to provide an apparatus for spectrographic .analysiswhich is direct reading and eliminates the calculations that arenecessary in making the same determinations by standard photographicprocedures.

Other objects will be recognized from the following description.

The invention will be described with respect to the accompanyingdrawings in which Fig. 1 is a schematic side elevation of an apparatusin accordance with the invention; Fig. 2 a schematic shown in Fig. 1,with the photocells andv circuits omitted; Fig. 3 a schematic sideelevation of an apparatus in accordance with the invention for thepurpose of illustrating primarily one mode of associating photocellsadjustably with the focal surface; Fig. 4 a sectional viewtaken on lineIV-IV, Fig. 3; Fig. 5 a fragmentary view of the line-selecting means ofthe photocell carrier shown in Figs. 3 and 4, viewed from inside theapparatus; Fig. 6 a sectional view taken on line VI--VI, Fig. 5; Fig. "la wiring diagram ofa circuit for supplying power to the photocelis; Fig.8 a wiring diagram of a unit that may be used to measure the energy ofselected wave lengths emitted by excited samples, in accordance with theinvention; Fig. 9 a schematic showing and wiring diagram of means forautomatically stopping a. determination; Fig. 10 a fragmentary viewsimilar to Fig. 5 showing a modied type of line-selecting means, withphotocells indicated dschematically; and Fig. 11 a plan view of Fig. 10.

-In accordance with this invention, and as has the dimculties antildisadvantages of the photographic method of analysis, can be operated byvary greatly and integrating the outputs of the cells over a selectedtime interval, at the end of which the indicating means are renderedinoperative, the amounts of elements to be determined are had directlyby the ratio of the energy count of each' such elementv to that of theinternal standard.

The invention may be described further in con- --nection withtheschematlc showings of Figs. 1

and 2, which represent apparatus conforming to it. Means are providedfor connecting a suitable source of electric current to a pair of elecofwhich may be the sample 3, suchas a block of alloy whose analysis isdesired, mounted in a suitable holder. In the embodiment shown oneelectrode is connected to the sample and the other is spaced from thebeen customary in the art, a sample is excited by appropriate means anda beam of the resulting radiation is passed by a beam-selecting means toa, light-dispersing means. Instead, however, of photographing theresulting spectrum and determining the density of lines characteristicof the elements to be determined, the lines, or narrow bands of spectralenergy, to be measured are passed, through line-selecting means locatedon the focal surface, to photocells which are used to measure -directlythe intensity of such lines. Thus we obtain directly measurements of theenergy of characteristic lines, or bands, due to excitation of elementspresent in the sample and thus we avoid the time-consuming features andsources of inaccuracy of the photographic spectrographic methods.

In accordance with a. further aspect of our invention the energy-outputsof the photocells are applied to actuate indicating means which thusrecord, or measure, the light energy received by' the photocells. Byrendering all of the counting means inoperative at a selected time, asby opening their circuits or the photocell circuits. the ra'- tios ofthe energy counts shown by the indicating means give the ratios of theelements determined.

sample. A member d, provided with a slit opening 5, selects a beam 6 ofthe energy emitted by the sample under the influence of an electricaldischarge sprung between specimen 3 and electrode 2 and passes it to alight-disperslng means which, in the preferred embodiment and as shownin Figs. 1 and 2, comprises a spherical grating 1.

Although these elements may be disposed in various ways, e. g., in themanner familiar in the art, we prefer to mount the grating 1 so as todirect the resultant spectrum, vshown schematically by the spectrumlines in Fig. 1, upon a focal surface 8 which, as shown by the drawings,lies in a substantially vertical plane which passes also through thesample, slit member and dispersing means. This is in contrast with thehorizontal optical plane that has been essential with photographicapparatus of this type because of the necessity for having the focalsurface convenient to the operators reach. lThat is unnecessary in thepresent invention, and the use of a vertical optical plane results in animportant saving 0f Space. Thus, with one particular instrument built inaccordance with this invention there is required a iioor space of only 8feet 4 inches by 15 inches, whereas the same instrument with the opticalplane horizontal would occupy a space 8 feet 4 inches by about 4 feet.In other words, as compared with the customary mounting required by thenecessities of photography, this preferred mounting affords a spacesaving of about 220 per cent.

In virtually heretofore, the practice has been to mount the slit andgrating so that they and the focal surface lie on the sc-called Rowlandcircle. We prefer, however, to mount these elements as described in acopending application of Joseph Geiner and Edwin R. Millen, filedconcurrently herewith, Serial No. 535,156. In accordance with theinvention of that application the slit lies closer to and the focalsurface lies farther from the grating than their Rowland circlepositions. In this manner the spectrum is spread out because thedispersion increases tance from the grating to the focal surface so thatit is unnecessary to use extremely -ne gratings or long focal lengthgratings to obtain the necessary dspersing and resolving power.

The apparatus provided by the invention includes a plurality ofphotocells 3, Fig. 1, mounted for adjustment along the focal surface 8,and the photocells take the place, of course, of the camera ofconventional instruments. In the embodiment shown adjustment of thephotocells is accomplished by providing a track T, Fig. 3, which is soshaped that its outer surface, over winch photo' all gratingspectrographs as made in direct proportion to the discell carriages maybe moved, positions the photocell slits, described later, exactly on thefocal surface of the spectrograph. Fig. 3 represents a side view of suchan instrument and it is intended primarily to illustrate the method ofmounting the photocells. The apparatus shown comprises essentially alight-tight box I0 having an enclosure II for receiving the samplespecimen, a slit member I2 adjustable in width of opening andtelescopically mounted for adjustment with respect to a sphericalgrating carried within a portion I3 of the light-tight box I 0.Enclosure lII is, of course, provided with an opening Afor emission ofthe energy from the excited sample. A rearward extension I 4 of the boxI0 receives the track -T which positions the photocell slits'on-the)focal surface. l' l Although the photocells may be mounted adjustablyin various ways, We havefound that sate isfactory results are to be hadby the construce tionillustrated in Figs, 3 to'6. The photocells-9 aremounted in light-tight containers I5 which are connected rigidly tocarriages I6 in the form of metal bars that extend across the openingI-l of the extension I4. 'Ihe carriages i6 are U- shaped, as seen inFig. 4, with their legs enclosing the outer edges of metallic members I8which are connected in any suitable manner to the framework of theinstrument housing and which form the trackiT. The carriages may beclamped rigidly in any desired positionfby clamping angles I9 which areheld in placeby lock screws `2li which extend through the carriage andinto the clamps Iii.l For ease of adjustment to dii-ferent positionsthere is provided at each end Aof each of the carriages I6 a pinion 2|which meshes with a rack 22 rigidly connected to member I8, the pinionsbeing rotatable by knurled handles" 23 which extend laterallyof thecarriages. The carriages can be movedl by loosening screws 20'androtating pinions 2i through handles-23, and when in desired positionthey are vlooked against movement by tightv eningscrews 20.

Each of the carriages is provided with abore 24 Which-:is surrounded-bythe neck 25 of the photocell housing I5 -for passing energyof selectedwave length to the photocell il,v andthe carriage is provided also witha line-selecting means which lies exactly on the focal surface so thatwhen in adjusted position the -photocell will receive the energy of apredeterminedv Wave length, In the embodiment of Figs. 4 to 6 this isaccomplished by a pair of plate members V2i? connected to the inner sideof carriage member I.6- and extending across opening 24. At theiradjacent edges plates 26 are provided with knife edges :21, Fig. 6, forthe purpose of isolating the particular relatively narrow band of energydesired from the rest of the spectrum. This serves also to diiracttheenergy passed through the slit so that the energy tends to cover amaximum area of the cathode of photocell Q Since the greatest efficiencyand accuracyare obtained when the entire cathode surface is illuminated;the cell is mounted far enough behind the slit to eifect this, asindicated particularly by Fig. 4.' It will be understood lthat in Fig.4'the photof cell is shown conventionally for clarity of il: lustrationbut that in practice the cell will be' positioned lengthwiseoflcontainer I5 so that the cathode will lie parallel to the slit. f

-In 'rumentsfof this typewill always embody atleast two, and commonlyatleast 3 or 4, photo-a c'ell'sf; depending'upon the complexity of thema? 6 elements to be determined. With the cell mounting just described-it would accordingly be impossible Ito bring 'two carriages closerAtogether than a certain minimum distance prescribed by their dimensionsand those of Athe photocell housings. Consequently, it would'beimpossible to measure two spectrum lines lying closer "together than lacertain minimum distance, although that'may in some cases be desirable.Ifhis jdiiiicul'ty is overcome to a large extent by mounting thephotocells as indicated in1i g.3,` which'shows an instrument providedwith fourphotocel'ls. Alternate housings I5 are provided with necksv 25which are relatively long, While .'theothr housings I5a have shorternecks 25a. `In thisway an adjacent pair of carriages may be' broughtcloser together than is possible where Iall of the 'cell housings areidentical. Thus, in jonev instrument a series `of housings have necks 25of such length that the photocells are 4 inches behind their slits,while the housings alternating' between them have necks of such lengththat the cells are 3 inches behind their slits. "The particularphotocells used have sensitive surfaces fe inch Wide and glass en-Velopes 11//8 inches in diameter so that in that instrument it ispossible to overlap two adjacent photocells until the edge of theenvelope of one is4 just in line with the edge of the sensitive surfaceof the next, in which position the two cells are at their minimumdistance of inch apart'. This Ipermits the simultaneous measurement ofvthe energy of lines ,close enough to each other for most purposes.

Because of the `minute amounts of energy involved, .the photocells usedin the practice of this inventionhare `preferably of the type known aselectron'multiplier tubes, suitable examples being R. C. A. l931or'C7045. These particular tubes require -a .constant voltage supply onthe order of 1000 to 1 200 volts, and their construction is such thatany variations in this high vol-tage powerv supply are represented inthe output by a ten-told magnilcation of the variation. To obtain outputreadings Aconsistent to l per cent it is'this necessary to regulate theoverall voltage variationto within sie of 1 per cent, Al.. though thismay be .done in various ways, the majority,` of circuits for thispurpose are either inadequatelyregumted or apply excessive voltagesV totheregulator tubes, with the result that their life is shortenedseriously. Hence we now prefer -to use -ior this purpose the circuitwhich is shownwinFig. .'7 and Which overcomes those difficulties, A highvoltage transformer 28u supplies. voltage-to a diode 29, such as R. C.A. 866, whose rectified output appears across lteij condenserl 30, .Ahigh gain voltage amplifier tube 3i, suchas R. C. A, 6J7, is connectedas shown sothat a'small change in the voltage output of the power supplycauses a change in grid bias and a corresponding change in platecurrent.' Since-thev plate current of tube 3! flows through resistorj32this change in plate current results in a change of grid bias of aregulator tube 33 whose plate-'cathode circuit is in series with theload circuit, represented by a series oftaps t corresponding in numberto the contact prongs of the photocell. If the output voltage increasesslightly the bias on the control tube 3| becomes'more positive, whichincreases the voltage drop' across resistor 32 and consequently'makes-'the grid bias of regulator tube srnrorenegative: 'This increasesthe effective resistance of the regulator tube and causes the outputvoltage to drop.

Contrasted with standard practice, the essential feature of this circuitis that no attempt is in reality made to regulate the entire outputvoltage. Since the dynodes of the multiplier cell drawv increasinglymore current as they PIOZI'GSS in order from the photocathode tothecollector, rigid control of the overall voltage would result inincreasingly severe defocussing action as the voltage steps would becomeincreasingly dissimllar with this action. The placement of tube ill (asthe reference voltage) which is suitably V. R. 105, requires highbleeder current ilow which is large compared to the photo-multipliercurrents and makes the step variations introduced by the multipliercurrents negligible. The voltage between tubes 3I0 and 33 is regulated,and since the remainder of the circuit is in series. the voltage acrossthe steps S-5 to S-ll is also of necessity regulated.

This displacement of the regulator section from the conventionalconnections permits using the tubes, particularly 3l, at appliedvoltages which do not exceed greatly the rated voltages under which thetubes normally operate and it thus results in improved performance andlonger tube life. The taps shown in Fig. '1 are connected to a. panelboard in known manner so that each of the photocells 8 receives powerfrom a single jack and cable.

Conditions within the sample excitation system can not be rigidlycontrolled, so that the intensities of the spectrum lines vary duringthe time of excitation. This introduces a variable in any system ofspectrographic analysis but we iind that its effect can be reduced to anamount which is inappreoiable for all practical purposes by measuringthe total energy emitted during a suitable time interval, i. e., byevaluating a time integral of the intensity over such an interval. Tothis end the output of each photocell is measured over a given timeinterval in any suitable way, but we prefer to accomplish this in themanner disclosed and claimed in the above-identiiied application. Asthere disclosed the photocell output is passed to a channel comprisingan amplifier unit 330, Fig. 8, an integrator unit 34, and a triggercircuit which actuates suitable vmeasuring or indicating means 360rdescribed more fully hereinafter. One such channel is provided for eachphotocell, as seen in Fig. 1. The amplier unit 330 may be any oi thevarious circuits adapted to that end but it should be tuned to thefrequency of the light passed to the photocell. With proper excitationof the sample the light reaches the cell at a frequency of 120 cyclesper second so that the ampliiler should be tuned to the same frequency.In this way microphonic effects and pickup from either internal orexternal sources are suppressed, thus obtaining more favorablesignal-to-noise ratio. Of course, -if the sample is excited with D. C.the light will not be modulated but in that event la chopper can beinterposed between enclosure Il and grating 1 to effect desiredmodulation in accordance with practice known in the art.

The'purpose of the integrator unit is to measure over a specific timeinterval the total energy to which its photocell is exposed, much thesame emulsion to give a summation of the total energy output over aspecific time interval so that different runs with a given 4material arerepro,- ducible. While various circuits might be used for this purposewe prefer to use an integrator unit 34, shown in Fig. 8. The alternatingcurrent output from amplifier 330 is coupled by a transformer 310 to arectifier diode 31, such as R.. C. A. 6J5, the D. C. output of which isimpressed across condenser 38 and resistor 39. This voltage is in turnapplied to a neon, or glow, discharge tube 42, suitably G.E. N.E.2,connected as shown `across a resistance 40 and a condenser 4I toconstitute a relaxation oscillator whoseoutput frequency varies as adirect function of the voltage across Bil-39. The discharge tube 42 thenres at intervals determined by the voltage supplied by 38-39 since thisdetermines the current supplied to the charge condenser 4I. Thefrequency of this relaxation oscillator is determined by the outputvoltage of amplifier 33D which is in turn determined at any instant bythe intensity of the light being measured. The dynamic range of the unitis limited by adjustment of the time constant of 38-39 relative to thatof 4ll-4l, and for most purposes it is preferable that the time constantof the former shall be several times that of the latter. The strikingvoltage of tube 42 is about 70 'volts so that the lower limit of powermeasurement ordinarily would be about 5 milliwatts. Therefore, avariable resistance 45 is used to supply initial set biasing potentialto neon lamp 42 just below the striking potential, thus enabling themeasurement ofmicrowatt power levels.

In one integrator-unit that has been used satisfactorily, condenser 38has a capacity of one microfarad, and resistor 39 a resistance of onemegohm, while condenser 4l has a'capacity of 0.05 microfarad, andresistance 4i) a resistance of 5 megohms; In this circuit the timeconstant of 38-39 is one second, While that of 4l-42 is V4 second.Biasing voltage, say volts, is supplied to the integrator unit throughterminals 43 and I4. This circuit provides, as stated, means forintegrating powers of fractional wattage. It is capable of operatingover a wide range, e. g., from a lower linut of measurement of a fewmicrowatts to an upper limit of about one-tenth watt. Over this range ofoperation the output consists of a series of pulses which may be appliedto actuate a counter mechanism that counts and records the number ofimpulses and which thus acts as a measuring or indicating means to showthe energy received by the photocell. By associating with this circuit astandard time in, terval mechanism to measure accurately the timeinterval over which the counter is operated there is obtained a directreading of the total power over the time interval. The output frequencyci several such integrator units built for the purposes of thisinvention has been found by measurement to be an exact linear functionof the input voltage or, in other words, the output frequency variesdirectly as the square root of the input power.

as a watt meter measures the total electrical eny ergy consumed in acircuit with which it is assoelated. This unit takes the varyingelectrical output of its photocell and integrates it, and thus it servesthe same function as a photographic Each pulsation of the integratorunit output represents a certain increment of spectral line energycontent. It remains only necessary, then, to countthe total number ofpulses accurately to have a' summation of the spectral -line energy overthe entire measurement interval. This is accomplished conveniently bypassing the output oi` the integrator to a trigger circuit 35, Fig. 8.and which comprises gas nlledtriode tubes 4Q and 41, such as It. C. A.2050sorg-20v51. Power, say at 100 volts, is supplied to the .triggerunit through terminals 48-and49. As appears from the circuit shown inFig. 8; the triode tubes 46 and 41 form a trigger circuit with the tubes46 and 41 becoming alternately: conductors or nonconductors upon eachpulsation from the integrator unit 34. That is, each ofthe said gas lledtriodes fires on alternate pulses of the neon tube 42,

. Although the resultant alternate iiring of .tubes 46 and 41 may beused in various` ways to indicateor count the pulses of the integratorunit, weprefer to use that illustrated in Fig. 8, which isfa novelmechanism. As appears from Fig. S, the. counter comprises a pair ofgeometrically opposed magnets 56 and 5| and a second pair ofgeometrically opposed magnets- 52 and 53 aligned at right angles to andin the plane of magnets 50 and 5l. These four magnets thus constitutethe pole pieces of a motor which includes a specially shaped rotor 54'mounted on a shaft-55. Magnets 58 and 5l are connected to be energizedupon the firing of triode tube 46 and to be de-energized when-triodetube 41 rires, while magnets 52,and 53 are in turn energized bythefiring of triodetube 4.1 and de-energized when triode tube 46 res.-Tl1usI rotor 54 is caused to rotate in stepwise fashion in conformity withthe pulsations of theintegrator unit. Chattering of the roter may beeliminated, if need be, by means oil a ratchet arrangement oroverrunning clutch, not shown. The number of movements of the rotor isindicated by connecting its shaft 55 to a suitable counting mechanism,as will be well understood, such as dial 36, Figs. 1 and 9.

The rotor is specially constructed as seen in Fig. 8. It comprises aspider having six equally spaced arms which are uniformly extended attheir ends to provideshoulder portions that progressively decrease incross sectional area to a sharp nose, as shown. In this way the magneticreluctance of the rotor increases. forl rotation in one direction anddecreases for rotation in the other direction. The result is that, asseen further from Fig. 8, when two aligned armsv are aligned with a pairof themagnets, the next adjacent pair of aligned arms in the directionof rotation lie with their nosesclosely adjacent to the other pair ofmagnets, which will be energized next, i. e., close to magnets 52 and 53in the drawing. Thus, when triode tube 41 fires the rotor is positivelycaused to rnove in clock-y wise direction, there being little tendencyfor it to rotate countercloclwise. When tube 41 has red the arms whichare at 60-240 in Fig. 8, will have moved so thatthey are in alignmentwith magnets 52 and 53 and at the same time the arms which are at120-300 in Fig. 8, will have been moved so that their noses lie adjacentto magnets 50 and 5I for being drawn into alignment therewith the nexttime triodev tube 4 6 l nres.

.'In the use of the apparatus described a iilm or other suitableemulsion used for spectrographic analysis is positioned along the trackwhich forms the focal surface, an appropriate sample is excited and theemulsion is exposed in accordance with standard practicein the art. Thisspectrogram gives a scale for positioning the photocell carriages in thecontinued use of the apparatus provided by the invention. Inmaking ananaly-j sis, then, thecarriages are moved soy that the slits of thephotocells admit the desired spectral cord the same number energy,whereupon the.sample 'is excited with power supplied to the variousunits.

For direct reading, without the necessity for extended computation, theinstrument is operated as follows. Assuming that samples ofaluminum-copper alloys are to be analyzed for copper, one photocell.carriage will be set to measure an appropriate copper line and anotherwill be set to measure anA aluminum standard 1ine,` aluminum being takenas the comparison standard because it is the preponderant element sothat changes in aluminum content of various samples are relatively less,proportionately, than in the case ofl alloying .elements present. Thesample excitation and exposure conditions as well as `the opticalalignment,A will be substantially the same as in pre-existing Practicewith photographic spectrographs. With a given sample excited and withthe ampliiier and integrator controls set at random, eachphotocellchannel (amplifier, integrator, trigger, counter) will, in general,record a random-number of counts. If the exposure were repeated underthe same conditions and for the same length of time, it would beexpected that each channel would again reo f Counts. except for ran-`dom variations. The controls on the aluminum line channel are thenadjusted by trial and error until the average numberof Counts will besome even number, such asl 5,00. or 1000, the upper limit of this valueb eing determined by the fact that the counter should not be operatingat a rate too near its maximum, while the lower limit is dependentuponthepermisslble error. Thus, if the lower limit isZOOthere will be amaximum error of 1/2 per cent ifl acount is missed at either thebeginning' or the end ol an, exposure. The controls onthe copper channelare likewise adjusted so that a Variation in,- copper content oi thesample yields apracticable variation in the number of counts. The sampleto be analyzed is-then excited. and whenwthe counter of the aluminumchannel has counted the predetermined number, say 500l or 1.000, thecircuits are opened. The copper content of these alloys will be relatedby a linear, or at leastby some regular, function to the number ofcounts so that by analyzing a series of samples of known copper contentthe dial of the C Opper channel counter can be calibratedto readdirectly in percentage of copper when the aluminum counter has countedits predetermined number of counts.

In this way direct readings may be had in a very few minutes, which is asmall fraction of the time required by photographic methods. Thus, ourinvention extends the utility of spectrographic analysis, as by makingit highly useful for control work, e. g., in alloy production, whererapidity of analysis is vital, A llthe more is this the casebecause theresults can be read directly from calibrated dials, and persons of notechnical, training or experience can perform routine analyses easilyonce the intrument has' wheel 65 mounted on a shaft. 66 which alsocarries,

acam 61 anda dial 36. Cam 61 isprovlded with a notch 69, and riding o nitsperiphery is a cam follower 'l0 which controlsfthe position of acontact Aspring 1| with respect to a second contact Spring 12 inthepower supplycircult. When cam follower` 1Q lies in the notch 89,contact spring 1| is When cam 61 is rotated so that cam follower 10 islifted out of notch y|59, spring 1| ,is also lifted and an electricalcircuit is completed', between con-` tact springs llland 12. Cam 61 ispositioned on shaft 66 so that its notch is aligned, with respect todial 3 6, so that Athe circuit between contact springs 1| and 12 isclosed at any dial reading betweenA 0 and the full scale reading but isabrupt-` ly opened when the Vshaft 66 rotates dial 36 iust past the fullscale reading position.

The functioning of this component of the entire structure then is asfollows. Before the start of a determination, shaft 66 is manuallyrotated until dial 36 reads 0 on its index 14.-

This, of course, rotates cam 61 to aposition where cam follower 10, andhence contact spring 1|',

12 size ofthe part of the original surface which is :obstructed by themirrors. Tracks comparable to -track I8 can then be set along each ofthe two new focal-surfaces, and photocells 8a and 9b, Figs. 10 and 11,held in carriages comparable to carriages I6 can be positionedtofreceive energy in beams are in the raised position. 'I'he circuitbetween 20 contact spring 1| and 12 is thus completed. In order thatworm wheel 65, in mesh with worm 6I, will not prevent such preliminarysetting, due

to the well known fact that a worm wheel of nor--I mal pitch cannotrotate a matching worm, a slip clutch 13 is provided between shaft 66and worm wheel 65.` With dial 3`6 .set at zero and cam 61 set to`complete circuit 'M -12, the run is started. Rotor 54 rotates as afunction of energy received at the standard line electron multiplier 9.When suiilcient energy has been received at standard electron multiplierv9, dial 36 will have rotated to a position-just past the full scalereading and cam follower 1|! will have dropped into notch 69, whichopens'circuit 1 |-12 and, through any appropriate electric circuit,stops further operation of all counters, including the standard lineone, by interrupting the power supply to them. The ratio of energyreceived at anyother electron multiplier to that "received at thestandard multiplier will then be indicated by the extent lof rotation ofthe dial of that second .multiplier rotor, since the standard rotoralways rotates a standard number of revolutions. e Y

Various.modiilcations are, of course, permis-` 6| and 62, respectively.These beams can be as close together in wave length as need be. or evenidentical. Y Y

According to the provisions of the patent statutes. we have explainedthe principle, construction, and mode of operation of our invention andhave illustrated and described what we now consider to represent itsbest embodiment. However, we desire to have it understood that, withinthe scope of the appended claims, the invention may be practicedotherwise than as specid cally illustrated and described.

We claim:

1. In a spectrograph, the combination of means for excitation of asample, means for dispersing light from the sample to form a spectrum, a

member having a slit disposed between said sample and spectrum-formingmeans, at least two selecting members disposed on the focal surface ofsaid spectrum-forming means and adjustable over said surface forselecting and passing individually predetermined narrow bands ofspectral energy received at said surface from said spectrum-formingmeans, a photocell associated with each of said selecting members formovement therewith and for receiving substantially simultaneously'lightenergy passed by the members, an electric circuit associated with eachof said photocells lfor measuring the intensity of light energy passedby its selecting member and including indicating means to register theenergy output of the photocell, and means associated f with one of saidindicating means to open the sible and willoccur to those skilled-in theart. 'L For example, the excitation of the sample can be controlledmanually .or automatically, andv likes wise the standard ycounter may beused for control, as by providing a cam which at the prel scribed numberof counts operates a micro-switch to open the various power circuits, orsuch con#- trol may be achieved otherwise or even be manual. Also,otherl circuits than shown in Fig. 8

may be used, `aswell asy other measuring and indicating means in placelof theV counter described, mechanism vand circuits for these pur-- posesbeing known inl the art. Likewise, the invention is not limited todetermining a single constituent although itwasdescribed for sim#plicity with vreference to determining the con- 1 tent of copper inanaluminum-copper alloy the same principle andmode of operationappliesto more complex systems.) Thus l a shows three photocellchannels, and'Fig. Elfshows` four. i

If for anyjreason ,itvv isr necessaryffto' measure j circuits of all ofthe indicating means when it indicates a predetermined value in responseto the photocell associated therewith.

2. In a spectrograph, the combination of means for excitation of asample, means for dispersing light from the sample to form a spectrum, amember having a slit disposed between said sample exciting meansand saidspectrum-forming means, said sample, spectrum-forming means l.and slitbeingrdisposed in a substantially horizontal-'plane and co-operating toform avertically extending spectrum, at least two selecting membersdisposed on the focal' surface of said spectrum-forming means andadjustable over said surface for selecting and passing individuallypredetermined narrow bands of spectral energy received at said surfacefrom said spectrumforming means, an electron multiplier photocellassociated with each of said selecting members with each of saidphotocellsfor integrating, while lines closer vtogetherQth'an,ispossible'withthe -foregoing construction, ,this. may/be done. by.

means of two" mirrors 160' with their surfaces form ing an angleofLfSQfand 'sett closer ,tothe grating y thanA the focal rsurfacef in'As'uch'ia `,position that j each mirror makesafi angle"' with''the'optical axis. These mirrorsv effectively splitpart' ofthe 4 focalsurface into two parts, 'eachbingjhali the the circuit is in'operation,the intensity of light energy lpassed to it, each said circuitincludingv counting ,means to register the energy output 0f lAthe'rphotocell of its circuit, and means associated withone of -saidcounting means operative when it indicates a predetermined value inresponse to the photocell associated therewith to render all 'fof ,thecounting means inoperative.

3.1..A'spectrographaccording to claim 2, said -`meansf,,associated withone of said counting means for rendering all of the counting means in-'foperatiye'being means to open all counting means circuits.

of the said 4. In a spectrograph, the combination of means forexcitation of a sample, means for dispersing light from the sample toform a spectrum, a member having a slit disposed between saidfsample andspectrum-forming means, at least two selecting members disposed on thefocal surface of said spectrum-forming means and adjustable over saidsurface for selecting and passing individually predetermined narrowbands of spectral energy received at said surface from saidspectrum-forming means, a photocell associated with each of saidselecting members for movement therewith and for receiving substantiallysimultaneously light energy passed by the members, an electric circuitassociated with each of said photocells for measuring the intensity oflight energy passed by its selecting member and including indicatingmeans to register the energy output of the photocell, and meansassociated with one of said indicating means and operative when itindicates a predetermined value in response to the photocell associatedtherewith to render all of the said indicating means inoperative.

AIKEN W. FISHER.

WILLIAM B. WARREN.

REFERENCES CITED The following references are o1 record in the le ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES An Investigation of theProperties and Applications of the Geiger-Muller Photoelectron Counter,an article by Duiendack et al. in J ournal of the Optical Society ofAmerica, for January 1942; pages 8 to 24 cited.

